Positive photoresists are currently being pushed to their limits to develop new processes for fine line lithography. Much of this processing is done in high temperature environments or with processes which generate a great deal of localized heat. Other processes use highly reactive free radical or ionic species. Further, current requirements demand that this processing be accomplished with minimal degredation of the resist image. All these requirements put increasing demands on the thermal stability of photoresist products. In view of these demands, a more complete understanding of the thermal chemistry of resist products would be of beneficial interest to those utilizing or designing these processes. This is of major importance in the development of new equipment, materials, and processes; especially where considerable heat is generated in the presence of resist materials. This paper attempts to provide a basic background to develop this understanding. It provides a description of the effects of thermal processing on positive photoresists. The paper discusses the thermal chemistry, particularly at temperatures above 100 degrees C, of the major positive photoresists used in microelectronic applications. The major emphasis is placed on common positive photoresist products containing diazo-oxide PAC and novolak resin. The paper gives a brief description of composition of the resists, and then an analysis of the thermolysis products at various temperatures. Analysis of the thermal reactions of the PAC, resin, and resist are given. The chemistry of the bulk films as well as the localized surface effects are examined. Finally, the effects of thermolysis are related to their impact on subsequent processing steps.